There is known a method disclosed in JP-A-2006-170183 as a method of adjusting ignition timing by use of a cylinder pressure sensor. According to the method disclosed in this patent document, a combustion ratio at given timing is calculated based on an output value of the cylinder pressure sensor. In addition, combustion start timing, i.e., ignition timing in a combustion chamber is adjusted so that the calculated value may coincide with a target value. Specifically, the ignition timing is adjusted so that the combustion ratio may be 50% at eight crank angle degrees after top dead center. It is known that when the ignition timing corresponds to MBT, the combustion ratio is 50% in the vicinity of 10° after top dead center. The method disclosed in the above-mentioned patent document is a method for achieving ignition at MBT by feeding back the output value of the cylinder pressure sensor. As a specific feedback, an ignition-timing calculation method is disclosed of calculating a deviation between a combustion ratio calculated from the output value of the cylinder pressure sensor and its target value and inputting the calculated deviation into a proportional-integral circuit.
Incidentally, although ignition timing is the same, a combustion state in a cylinder varies depending on the operating conditions of an internal combustion engine, particularly, on a variation in engine speed. Therefore, even if the combustion ratio coincides with the target value at certain ignition timing, when the operating condition varies, the combustion ratio calculated from the output value of the cylinder pressure sensor is varied by the variation of the combustion state to cause a deviation between the combustion ratio and the target value. The method disclosed in the above-mentioned document allows the proportional-integral circuit to correct the ignition timing so as to eliminate the deviation. However, during from the variation of the operating condition until the ignition timing is optimized again, the combustion state deviates from the optimum state. That is to say, the method disclosed in the above-mentioned document has room for improvement in the controllability of the combustion state, specifically, in the controllability in situations where the operating conditions are variable.